The invention relates generally to membranes and methods for separating a gas from a gas stream, and particularly for separating CO2 from a gas stream.
The separation of CO2 from a gas stream is a critical step in the reduction of greenhouse emissions from fossil fuel-based combustion processes. An amine process is used commonly in power plants to scrub the fuel or exhaust gas stream. However, this approach is both energy and capital intensive because the amine process involves cooling the gas stream before scrubbing. Membranes capable of separating CO2 at temperatures above 200° C. could be used in lieu of the amine process in existing plants. They could also be used in advanced integrated gasification combined cycle (IGCC) plants to improve efficiency. To be successful, a membrane must meet two sets of requirements. First, the membrane must be able to selectively separate CO2 from a gas stream. In particular, it is desirable to separate CO2 from H2 in the fuel gas stream or to separate CO2 from N2 in the exhaust gas stream. To achieve separations with a porous membrane, it is often preferable to have reverse selectivity. Reverse selectivity is selectivity in which the heavier gas is enriched relative to the level expected for Knudsen selectivity. Second, the membrane must have an operating temperature above 200° C.
For example, a high temperature membrane having reverse selectivity in separating CO2 and H2 must exhibit mechanical and functional stability up to 500° C. and CO2/H2 selectivity greater than 10. In addition, CO2 permeabilities of at least 1000 Barrer are desirable. There are no membranes currently available that meet these requirements.
Since CO2 is heavier than the other components of interest in the gas stream, Knudsen diffusion is not a viable mechanism for separation. Knudsen diffusion describes the flow of gas through a membrane in which the pore size is small compared to the mean free path of the gas. The Knudsen diffusion rate is inversely proportional to the molecular weight of the gas. A membrane relying only on Knudsen diffusion would have a CO2/H2 selectivity of 0.21. Instead, transport must occur through alternate mechanisms that enable the desired selectivity. For example, the most promising polymer membranes are based on a facilitated transport mechanism in which CO2 is selectively transported via amino groups. Those membranes exhibit selectivity of about 10 and permeability of 2000 Barrer at 180° C., but performance rapidly degrades above 180° C. due to dehydration of the membrane. Therefore, polymer membranes are not suitable at higher temperature.
Porous inorganic membranes have the capability for high temperature applications, and selectivity can be endowed through the mechanism of preferential adsorption and surface diffusion of CO2 along the pore walls. Based on this approach, CO2/N2 selectivity of ˜10 have been reported for zeolite, silica, and activated carbon membranes with permeabilities as high as ˜104 Barrer (at room temperature). Recent efforts to develop reverse selective membranes using this strategy have resulted in silica membranes having a measured selectivity of ˜5 to 7 with permeability of about 1000 Barrer at 40° C. (Moon, J. H., et al., Kor. J. Chem. Eng., 21, 477-487 (2004)). Up to this point, efforts to develop membranes with enhanced surface transport have focused on identifying a porous material which itself has suitable surface transport properties. The problem with this approach is the limited number of compositions available that satisfy both the structural requirement (well-defined pores) and the transport requirement (fast surface diffusion of CO2). Kusakabe et al. have prepared barium titanate (BTO) layers on porous alumina supports and found a CO2/N2 selectivity of 1.2 at 500° C. (J. Membrane Sci., 95, 171-177 (1994)). The expected selectivity from Knudsen diffusion is 0.8. However, the membranes contained structural defects in the form of 100 nm pinholes that limited CO2 selectivity.
Accordingly, there remains a need for membranes that can achieve CO2/H2 selectivity significantly higher than that achievable through Knudsen diffusion mechanisms at high temperatures.